Do this by sharing the thread which processes emails for that directory.

Fixes LP #574286

Updated to the latest zope.testing and backport the deprecation avoidance changes for zope.testing.doctest

 fixes lp:143013 on this branch

Update to new i18nmessageid to get the memory leak fix. The other changes in the 3.5.x series are all fully backwards compatible

In previous Zope versions string exceptions were used and all exceptions
re-raised. This doesn't work with each kind of exception, but we can still do
it with Unauthorized exceptions. This way the special handling for Unauthorized
exceptions works again, HTTPResponse._unauthorized is called again. This is now
done after rendering to make sure we don't break logging and custom views.

Such cookies violate RFC 2965 / 2616.

Fixes LP #563229.